1. Field of the Art
An arc furnace transformer must be designed for very large secondary currents. For large arc furnaces the currents may amount to 80 to 100 thousand ampere.
2. Prior Art Statement
Traditionally arc furnace transformers have been designed with open secondary phases and the delta connection of the phases has been located outside the transformer tank. In that manner current compensation has been obtained by passing the forward and return current conductors close to each other, so that the magnetic fields become small. This has made it possible to cope with the current losses and heat problems. However, such outside delta connection becomes very bulky because of the large current intensity and poor cooling conditions. Short insulating distances in air combined with dust deposits likewise involve a permanent danger of flashover between the conductors.
In recent years it has become more usual to place the delta connections within the transformer tank, where cooling and insulating conditions are ideal. On the other hand this involves the necessity of passing heavy currents without compensation through the wall or cover of the tank, which in turn requires expedients for preventing high eddy current losses and heat problems. Therefore, in many cases there has been used a separate duct panel of insulating material or of non-magnetic material with good electric conductivity. This panel has been attached in a corresponding opening in the tank wall by means of fastening elements. A main weak point of such a structure is the sealing between tank and panel.
In operation it will always be a problem to obtain even load in an arc furnace, since the conditions in the furnace are constantly changing. It has therefore been desirable to make it possible to control the electrode voltages (phase voltages) individually and independently of each other. In order to make this technically possible, secondary phases of the furnace transformer must be star connected. Such connection can also be effected within the transformer by means of bars like in the case of a delta connection. But even in this case the relatively complicated bar system involves the same risk of high eddy current losses and heat problems.
According to the present patent application these difficulties are avoided by a transformer designed in accordance with the main claim of the application.
The neutral point plate with screen plate as taught by the present invention replaces the complicated part of the bar system for connecting the secondary phases of an internal star system. The screen plate between the neutral point plate and the tank wall will effectively prevent induced currents and reduce temperature problems in the wall of the tank.
The advantages of this invention consist in that the neutral point plate as compared with a conventional bar system makes the connections between the secondary windings and the wall ducts simpler and shorter. This results in lower short circuit reactance and loss. The screen plate chokes efficiently the magnetic field from the neutral point plate, so that the tank wall in its entirety can be made as a welded structure. A duct panel of special design will therefore not be necessary. This avoids the leakage problems associated with a duct panel with flanges.